1. Field of the Invention
The present invention relates to a diamond hetero-junction rectifying element comprising a diamond film and a film of either silicon carbide or silicon, and having an excellent heat resistance.
2. Prior Art References
Diamond is a very hard material and has a high thermal conductivity as well as an excellent resistance to heat, radiation and chemicals. Recently, it became possible to prepare a diamond film by chemical vapor deposition (CVD). A speaker diaphragm and a heat sink for an electronic device are being developed. Diamond free from impurities is insulating, but diamond can be converted to p-type semiconductor by doping. The band gap of the p-type semiconductor is as large as about 5.4 eV. Moreover, its semiconducting characteristics persists at high temperature above 100.degree. C. Its dielectric breakdown voltage is about 30 times higher than that of silicon (Si). Therefore, there is an interest in the application of diamond for high temperature and high power electronic elements. A p-n junction rectifying element is one of the targets of development.
In order to obtain a p-type semiconducting diamond, B is doped in the diamond film. To obtain n-type semiconducting diamond, Si or P is doped. However, n-type semiconducting diamond having good electrical properties have not been successfully obtained. Therefore, a hetero-junction using a n-type non-diamond semiconducting material is considered. For example, a rectifying element comprising p-type diamond and n-type amorphous silicon (hereinafter referred as to a-Si:H) has been proposed (Hideo Kiyota, Masahiro Yoneda, Hiroshi Izumiya, Hideyo Okushi, Ken Okano, Tateki Kurosu and Masamori Iida, Japanese Journal of Applied Physics, Vol.31, No.4A, Part 2, pp. L388-L391, 1992: hereinafter referred to as prior art references 1).
FIG. 11 is a cross-sectional view showing an a-Si:H/diamond hetero-junction rectifying element according to the prior art reference 1. Here, a p-type polycrystalline diamond film 3 is formed on a single crystal silicon substrate 4, a n-type a-Si:H layer 2 is formed on the p-type polycrystalline diamond film 3, and an ohmic electrode 1 made of Mg is selectively formed on the n-type a-Si:H layer 2. An ohmic electrode 5 made of Au is formed on the back surface of the single crystal silicon substrate 4. In the prior art reference 1, a rectification ratio of 10.sup.2 to 10.sup.3 was obtained from room temperature to 100.degree. C. In other prior art rectifying elements having a p-type diamond film and a n-type a-Si:H layer, a rectification ratio of 10.sup.2 has been obtained at room temperature.
FIG. 12 shows a prior art p-n-p type hetero-junction bipolar transistor (HBT) comprising a diamond film and a cubic silicon carbide (.beta.-SiC) (Japanese under Provisional Publication sho 64-55862, hereinafter referred to as prior art reference 2). In this diamond HBT, a p-type .beta.-SiC layer 7 as a collector is formed on a single crystal diamond substrate 6. A n-type .alpha.-SiC layer 8 as a base is selectively formed on the p-type .beta.-SiC layer 7. Further, a p-type polycrystalline diamond film 3 as an emitter is selectively formed on the n-type beta-SiC layer 8. Ohmic electrodes 9 made of Au/Mo/Ti multi-layers are formed on each of the p-type .beta.-SiC layer 7, the n-type .beta.-SiC layer 8 and the p-type polycrystalline diamond film 3.
The a-Si:H layer 2 used in the prior art reference 1 as the n-type semiconductor, however, is not suitable for use at high temperature because hydrogen is released at temperatures higher than about 350.degree. C.
In the prior art reference 2, the .beta.-SiC layer 8 having an excellent heat stability is used as the n-type semiconductor, and therefore there is no such problem. In this prior art reference 2, the diamond layer is formed on the n-type .beta.-SiC layer 8 by the microwave plasma CVD method (microwave power: 400 W, source gas: C.sub.5 H.sub.8 :0.2%+B.sub.2 H.sub.6 :0.5 ppm+H.sub.2 :99.8%, gas pressure: 30 Torr). However, this method provides polycrystalline diamond film. Such a film contains many grain boundaries and crystal defects. Therefore, if the element is operated in air at high temperature, the diamond film is oxidized and graphitized gradually along the grain boundary.
If the p-n junction is formed using a p-type polycrystalline diamond film 3 and a n-type .beta.-SiC layer 8, grain boundaries crystal defects cause a current leakage in the reverse bias conditions (positive bias to the n-type semiconductor).
As shown in FIG. 13, no such problem occurs, if a rectifying p-n junction element is provided by a single crystal diamond substrate 6 and a p-type single crystal diamond film 10 formed on said single crystal diamond substrate 6. In FIG. 13, the p-type single crystal diamond film 10 is formed on the single crystal diamond substrate 6 and the n-type .beta.-SiC layer 8 is selectively formed on the p-type single crystal diamond film 10. An ohmic electrode 11 is formed on the n-type .beta.-SiC layer 8 and an ohmic electrode 12 made of a Au/Ti bilayer is formed on the p-type single crystal diamond film 10.
Since the single crystal diamond is expensive, the manufacturing cost is high. In addition, the surface area of the commercially available single crystal diamond is limited to about 5.times.5 mm.sup.2. Therefore, mass production of single crystal devices is not practical.